The prior art is already aware of combined writing and computing instruments such as U.S. Pat. No. 3,568,923 to Chapman and combined writing instruments and extensible rulers such as U.S. Pat. No. 1,607,097 to Murphy. One problem with prior art devices of these two types is that they have been relatively expensive to produce. Another problem associated with such devices is that it has not been possible to view the scales carried by the devices in dim light.